1. Field of the Invention
This invention relates generally to reducing corrosion of waterwall tubes in a coal-fired boiler that has a low NO.sub.x system (a low NO.sub.x boiler). More particularly, this invention relates to a method for reducing corrosion of waterwall tubes caused by the presence of FeS deposits.
2. Description of the Related Art
Clean air regulations require a reduction in the flue gas emissions of NO.sub.x compounds from coal-fired boilers. One method for achieving such a reduction is by the use of a low NO.sub.x burner system that employs deep staging. Through the use of deep staging, NO.sub.x production can be reduced by initially burning the coal with insufficient quantities of air in the lower furnace or primary combustion zone. This results in an insufficient concentration of oxygen in the flue gas to combine with nitrogen to form NO.sub.x compounds. As the flue gas passes from the primary combustion zone into the upper furnace area, additional air, known as overfire air, can be added to complete the combustion process.
In boilers operating without low NO.sub.x burner systems, and therefore in the presence of excess oxygen, low alloy or carbon steel waterwalls are protected from corrosion by the formation of an iron oxide scale, typically Fe.sub.3 O.sub.4, on the surface of the waterwall tubes. This scale is dense and essentially impermeable to gas and adheres strongly to the surface, thereby making fireside corrosion of the waterwall negligible. However, with boilers equipped with low NO.sub.x systems, the lower furnace maintains a reducing environment due to the lack of oxygen, and protective iron oxide scale does not form readily. Instead, the waterwall tubes exhibit increased corrosion caused by FeS deposits. This results in low NO.sub.x boilers experiencing additional downtime and costs.
In view of the foregoing, there exists a need for a method to reduce corrosion of waterwall tubes in boilers that employ low NO.sub.x systems. In particular, there exists a need to reduce the very rapid corrosion caused by FeS deposits.